Wireless communications for the masses have historically been supported by cellular networks. In cellular networks, base stations are geographically distributed over a given coverage area and form cells in which cellular communications are supported. As a mobile terminal moves from cell to cell, communication support is transferred from one base station to another. If the mobile terminal is engaged in a call, adjacent base stations and the mobile terminal cooperate to perform a handoff from one base station to another in a manner imperceptible to the user. Cellular communications have generally been used to support calls using circuit-switched connections in association with a public switched telephone network.
With the evolution of packet networks and corresponding wireless access mechanisms, the availability and popularity of supporting voice-based calls over a packet network have dramatically increased. Such packet-based calls are often referred to as voice over packet (VoP) calls or voice over Internet Protocol (VoIP) calls, when the calls are supported by the Internet Protocol (IP). VoP calls can now be established between various communication terminals that support packet communications, as well as with traditional PSTN communication terminals via gateways, which support communications between the PSTN and packet networks. As a communication terminal moves from one local wireless communication zone to another, handoffs from one access point to another are made.
The access mechanisms for packet networks often provide local wireless access based on various wireless local area network standards, such as those promulgated by the IEEE's 802.11 standard and the Bluetooth standard. Wireless access points, which provide access to the packet network, can be geographically distributed to support contiguous communications zones for a given area. Local wireless communication zones are generally much smaller than cellular network cells and were initially used to provide Internet access for residential and business use. Now local, wireless communications are being used to replace wired local area networks (LANs) as well as support VoP calls in areas where cellular coverage is unavailable or unacceptable as well as in areas where an alternative to cellular access is desired.
Since calls can be supported by packet networks and cellular networks, there is a movement to develop dual mode mobile terminals capable of supporting cellular calls over the cellular network via cellular access as well as VoP calls over the packet network via local wireless access. When access for a call is supported entirely through either cellular or local wireless access, handoffs from base station to base station or from access point to access point are controlled using the handoff techniques for cellular or local wireless communications, respectively. When the dual mode mobile terminal needs to change from operating in a local wireless mode to the cellular mode or vice versa, mode switching must be performed. If a call is in progress when mode switching is necessary, continuity of the call must be maintained by facilitating a handoff from local wireless support to cellular support or vice versa.
Although numerous techniques are available for effecting mode switching between local wireless and cellular modes, the existing techniques for determining when to initiate mode switching are lacking. Existing techniques rely primarily, if not solely, on generic signal strength thresholds for signals received from the base station or access point. Primary reliance on the generic signal strength threshold are often unreliable due to the manner in which channel conditions can fluctuate throughout a local wireless zone or from one local wireless zone to another, especially communication zones near or inside buildings. Setting power thresholds for mode switching too high increases the risk of dropping a call, and setting power thresholds too low unnecessarily increases the number of mode switches. Accordingly, there is a need for a more efficient mode switching technique that minimizes unnecessary mode handoffs while ensuring communication support is not lost.